Conventionally, a technique is known which subjects a sound signal to filter processing using a head related transfer function (HRTF) filter for diffuse sound field, to obtain sound having an objective feature (e.g., see Patent Document 1). For example, in the technique described in Patent Document 1, out-of-head-localization, sound localization in front and back, is simply facilitated partially using the HRTF filter for diffuse sound field, in headphone listening.